A feature of fundamental importance for all living cells is the ability to respond to signals from the environment. Examples of this basic property are the detection of light by retinal cells, the induction of differentiation by growth factors or the movement of bacteria towards nutrients. Bacterial chemotaxis, the movement of bacteria towards attractants and away from repellents, serves as a model system to study how signals are transduced into the cell and processed into proper responses. The chemotaxis system of the bacterium Escherichia coli is one of the best characterized signaling systems. Crucial constituents of any signaling system are receptor proteins which are responsible for initial steps in the signal transduction process.

Bacterial chemoreceptors are transmembrane proteins which reside in the cytoplasmic membrane. While the receptor polypeptide chains are known to have two membrane spanning segments, the precise membrane boundaries have not been identified. Cellular signaling is commonly communicated in a phosphorylation cascade through protein kinases. Chemoreceptors recognize changes in the chemical environment, mediate transmembrane signaling and control the activity of an intracellular kinase.

Chemoreceptors are also involved in adaptation to constant stimuli, which is accomplished through covalent modification of the receptor. While chemoreceptors are thought to exist as homodimers, higher oligomeric states have been observed and suggested to be important for receptor function. However, a rigorous direct correlation between the different receptor functions and oligomeric state has not been established.

Aim of this thesis was to explore the location of the membrane boundaries of chemoreceptors and to investigate the relationship between oligomeric state of chemoreceptors and chemoreceptor functions. The results of this work allowed to precisely define membrane boundaries and to identify a correlation between oligomeric state and function. Membrane boundaries defined by the experiments are located interior to boundaries expected from sequence. The results provide new information about chemoreceptor organisation and suggest an experimental definition of membrane boundaries for other transmembrane proteins. Thus far membrane boundaries have been determined experimentally only for few proteins and the technique employed in this study provides a convenient and minimally perturbing approach. Correlating receptor functions and oligomeric state provided novel insight, demonstrating that different receptor functions are mediated by receptor in different oligomeric states. In the course of defining the oligomeric state of receptors we adapted a novel technology which was recently developed to isolate receptor in a native environment. This technique will be of wide importance in studying transmembrane proteins.